1. Field of the Invention
The present invention relates to a semiconductor device having a hermetically sealed hollow structure.
2. Background Art
Since general-purpose high frequency semiconductor devices, including field effect transistor devices of compound semiconductor such as GaAs or GaN, etc., have rapidly become prevalent, there has been a great need to reduce their cost. In order to meet this need, low-cost molded packages have been adopted instead of conventional fully hermetic metal packages. However, the use of a non-hermetic package such as a molded package requires that the semiconductor device contained therein be moisture resistant in order to prevent various types of degradation due to moisture. A conventional method for providing the semiconductor device with moisture resistance has been to prevent infiltration of moisture into the semiconductor device by covering the surfaces of the semiconductor elements and various metal films on the principal surface of the substrate using a thick insulating film formed by plasma CVD, etc.
Insulating films formed by plasma CVD or the like, however, may tend to absorb moisture, depending on the conditions under which they are formed. Further, the thick insulating film, unlike a thin insulating film, may peel off due to stress change resulting from absorption of slight moisture by the film. This degrades the coverage performance and quality of the film at step portions associated with the configurations of the transistors of the semiconductor device. As a result, the thick insulating film is likely to transmit and absorb moisture, meaning that the film cannot fully prevent infiltration of moisture into the semiconductor elements. Various types of degradation have been found to occur due to such moisture infiltration. Further, the increased thickness of the insulating film results in increased capacitive components and hence degraded high frequency characteristics of the semiconductor device.
In order to solve this problem, a semiconductor device having a hermetically sealed hollow structure has been proposed in which semiconductor elements are hermetically sealed by bonding a cap wafer to the principal surface of the substrate using glass frit as adhesive, which is disposed on the principal surface of the substrate in such a manner as to surround each semiconductor element (see, e.g., Japanese Laid-Open Patent Publication No. H10-74855).